Get New Points from Angle Android

Question

I am trying to make a Re-sizable touch view which I made successfully. You can find code How to make view resizable on touch event.

It has 4 corners. You can re-size that rectangle by dragging one of corner. But now I want to enhance that logic and want to put rotation in that code. I successfully find angle when user touch center of one of the edge of rectangle. But now problem is I can't get new position of corners so that I can redraw that rectangle and rotation is possible.

Question is : How can I calculate 4 corners new position based on Angle?.

score 0 · Answer 1 · answered May 06 '13 at 11:16

0

If you know the angle by which to rotate then you don't need to calculate the rectangle's vertices. A simple way to do it would be as below

@Override
        protected void onDraw(Canvas canvas) {
            super.onDraw(canvas);
            canvas.save();
            canvas.rotate(60.0f);

            paint.setColor(Color.BLACK);
            paint.setStyle(Paint.Style.STROKE);

            canvas.drawRect(10, 10, 100, 100, paint);
            canvas.restore();
        }

answered May 06 '13 at 11:16

asloob

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I know that scenario to rotate canvas.. but then after points were on their old place. Next time I can't find new place of 4 corner points on canvas. This will not work my friend. – Chintan Rathod May 06 '13 at 11:32

Davide Cannizzo · Answer 2 · 2018-01-09T16:21:59.330

As I understood, you want to calculate new coordinates based on a rotation angle... It's probably simpler than you think:

x' = x × (cosα - sinα)
y' = y × sinα × cosα

So, you just apply this programmatically, considering where x' is the rotation result of x and the same for y' and y, and α is the rotation angle.

cosα and sinα functions are available in Java as Math.cos(α) and Math.sin(α), but attention: in Java, all trigonometric functions uses radians and not degrees as angle, so you can consider this: rad = deg * 180 / π Applicable as:

double deg = 45d; //Put instead your degrees
double rad = deg / 180 * Math.PI; //The radians convertion

Get New Points from Angle Android

2 Answers2